For a radio communication, an error correcting code is a technology indispensable for transmitting/receiving high-reliability information. A turbo code is becoming a focus of attention as a correcting code having a strong error correcting capacity in recent years.
A basic turbo coder is constructed of a parallel connection of a plurality of recursive systematic convolutional coders (RSC) through an interleaver as shown in FIG. 1 and outputs a systematic bit (information bit) X and parity bits Y1, Y2.
Furthermore, MIMO (Multiple-Input Multiple-Output) is becoming a focus of attention in recent years as a system for effectively realizing high-speed transmission using a limited frequency band.
The MIMO is a system which uses an array antenna for both transmission and reception to simultaneously transmit/receive independent signals on the same band using a plurality of eigen vectors. Using this MIMO makes it possible to expand a transmission capacity without expanding the frequency band.
There is also an on-going study for improving communication quality by combining a turbo code and MIMO.
Here, when the ratio between systematic bits and parity bits transmitted is equal among eigen vectors, the amount of improvement in communication quality increases as the respective eigen values become more uniform and the effect of improvement in communication quality decreases as the difference between a maximum eigen value and other eigen values increases.
However, in a conventional communication system, magnitudes of eigen values are not taken into consideration and the ratio between systematic bits and parity bits of each eigen vector always remains constant, and therefore when a difference between a maximum eigen value and other eigen values is large, the effect of improvement in communication quality using a turbo code decreases.
Especially in a situation such as a base station of a cellular system in which an antenna on the transmitting side is set high for the purpose of covering a wide range, a maximum eigen value becomes a value by far larger than other eigen values, and therefore it seems necessary to add certain improvement.